1. Field Of The Invention
This invention relates to a cutting tool tip for a machine tool.
2. Description Of The Prior Art
At present, cutting tool tips for machining workpieces formed of hard metals and alloys, such as hard steels, are normally produced from alumina and tungsten carbide-based materials. It is, however, found that these known tool tips have an undesirably short working life when used in certain onerous applications with this type of workpiece, particularly in single point turning (that is where the tool is held against rotation while the workpiece rotates), and in rotary turning where repeated interruptions are encountered in the turning operation. As a result of investigations which have been carried out to alleviate this disadvantage, it has now been found that a cutting tool tip having an improved working life as compared with the known alumina and tungsten carbide-based tips can be obtained by forming at least the cutting edge of the tip from a ceramic product consisting essentially of at least 75% by volume of a single phase compound having an expanded .beta.-phase silicon nitride lattice and the general formula Si.sub.6-z Al.sub.z N.sub.8-z O.sub.z where 0&lt;Z.ltoreq.5, together with a second phase containing a metal from the list yttrium, lanthanum, cerium and scandium, or other rare earth metal.